1. Field of the Invention
This invention relates to an optical pickup device, and more particularly, to an optical pickup device that uses ultra-resolution to lessen a spot size of a laser beam which is converged on an optical disk.
2. Description of the Related Art
An optical pickup that uses ultra-resolution to lessen a spot size of a laser beam which is converged on an optical disk is known. This pickup generates the ultra-resolution by a device that decreases light intensity at around a center portion, in cross section, of the laser beam. By the ultra-resolution, the laser beam is divided into a main lobe and a pair of side lobes which are positioned at both sides of the main lobe, and converged on the optical disk through an objective lens. As shown in FIG. 7, a minor axis (i.e., the width) of the main lobe is smaller than that of a normal laser beam (shown by the dashed lines), and relatively intense side lobes are produced at both sides thereof. The ultra-resolution is known, and disclosed in some publications: H. Ostenberg and J. E. Wilkins, "The resolving power of a coated objective," J. Opt. Soc. Am., 39 (1949) p553-557, and J. E. Wilkins "The resolving power of a coated objective II," J. Opt. Soc. Am., 40 (1950) p222-224.
FIG. 8 is a schematic drawing showing, as an example, an optical pickup device utilizing ultra-resolution.
Referring to FIG. 8, an outgoing beam out of a semiconductor laser 51 is collimated into parallel rays through a collimator lens 52. The collimated beam passes a beam shaping prism 53, first and second beam splitters 54 and 55, respectively, a quarter wavelength plate 56 and an objective lens 57, thereby being applied to an optical disk 100. A band-shaped light shielding plate 58 is disposed between the beam shaping prism 53 and the first beam splitter 54. The light shielding plate 58 decreases light intensity around a cross sectional center of the laser beam so as to generate ultra-resolution, thereby lessening a spot size of the laser beam converged on the optical disk 100.
The laser beam is reflected on the optical disk 100 and goes through the quarter wavelength plate 56. Then, part of the laser beam is reflected by the second beam splitter 55 and converged in a slit 63 of a slit plate 62 through a converging lens 61. The slit 63 cuts off side lobes in the reflected laser beam so that a first optical detector 64 receives only a main lobe (reproduced signal component). Thus, signal components of the side lobes are removed since they are unnecessary.
On the other hand, part of the laser beam going through the second beam splitter 55 is reflected by the first beam splitter 54. Then, the reflected laser beam is condensed through a second converging lens 71, and directed to and received by a detecting system for focusing error and tracking error signals. Namely, part of the laser beam from the second converging lens 71 passes through a third beam splitter 72 and converged on a second optical detector 73 that outputs detection signals. Focusing error signals are produced based on the detection signals from the second optical detector 73. The rest of the laser beam is converged on a third optical detector 75 that outputs detection signals. Tracking error signals are produced based on detection signals from the third optical detector 75.
Japanese Patent Publication (Kokai) No. 4-125826 discloses an optical pickup device having a similar structure to that of the device of FIG. 8. This device has a mirror on an upper surface of a member corresponding to the slit plate 62 of FIG. 8, to reflect side lobes, and directs these side lobes to a detecting system for focusing error signals.
Unfortunately, with the above two optical pickup devices, the slit plate 62 may cut off necessary components in the main lobe, thereby deteriorating a S/N ratio of the reproduced signals.
Moreover, with the above two optical pickup devices, the slit plate 62 must be very accurately disposed at a predetermined position in order to pass the main lobe through the slit 63, thereby necessitating a positioning work with high accuracy and taking much labor.